The present invention relates to a scanning electron microscope for observing a very small area of a sample, and more particularly to a scanning electron microscope which has a computer, such as a personal computer or a workstation, as a constituent element and which can be operated using its window system.
Electron microscopes are used for observing very fine structures of samples in various research and development fields. In general, evaluation of a sample using an electron microscope is performed by observing a SEM (scanning electron microscope) image on a display screen. However, as high performance personal computers and workstations are being reduced in price and are being widely used, a technology has been used in recent years, wherein SEM images are stored as a file of image data and the stored SEM images are evaluated with image processing, such as a filtering process, using a personal computer or workstation. An example of a system which provides storing and retrieving of SEM images is disclosed in Japanese Patent Application Laid-Open No. 1-8485 (1989) titled "Image retrieving and analyzing workstation". Further, there has been developed an electron microscope in which a personal computer or a workstation is used as a constituent element of the electron microscope to adjust its focus from an operating picture using a window system. The window system of a personal computer constitutes system software for providing and managing a standard window and parts, such as buttons. This system makes development of application software easier and provides an operator with a common operational environment. Therefore, the window system is becoming widely used.
FIG. 2 shows an example of a conventional construction of an electron microscope which has a personal computer as a constituent element.
An electron beam 7 emitted from an electron gun 1 is accelerated by an acceleration voltage, which can be variably set, and is focused on a sample 5 disposed on a sample stage 6 by a focusing electron lens system of an irradiation system 2, which is operated under control of an irradiation system controller 9. The focusing lens system may comprise a single focusing electron lens or a plurality of focusing electron lenses. The irradiation system 2 further includes alignment coils, which serve to correct the deviation of the axis of the electron beam from a correct axis thereof, and stigmater coils, which serve to correct astigmatism of the electron beam, the alignment coils and the stigmater coils being operated under control of the irradiation system controller 9. The electron beam is further deflected two-dimensionally by deflection coils of a deflection system 3, which are operated under control of a deflection system controller 10, and consequently the sample 5 is two-dimensionally scanned with the focused electron beam. The deflection system 3 further includes image shift coils, which are operated under control of the deflection system controller 10, so as to select a field of view of the sample 5, i.e., a position of irradiation of the sample 5 with the electron beam 7.
Scanning of the sample 5 with the beam 7 generates secondary particles 8, such as reflected electrons, secondary electrons and so on, i.e., particles characteristic of the sample 5, depending on the shape and the material of the sample 5. The secondary particles 8 are detected and amplified by a detector device 4, which includes a secondary particle detector and a bias and gain controller thereof, and then the signal is converted into a digital signal by an A/D converter 11 and stored in an image memory 13. An address control circuit 12 generates an address of the image memory 13 in synchronism with a scanning signal of the electron beam. The address control circuit 13 also operates to read out an SEM image stored in the image memory 13 with a television scanning speed and the signal is D/A converted by a D/A converter 20 to form a video signal. The video signal is displayed on a display 15 of the personal computer in real-time by being processed with a super-imposing circuit of an image composing means 17.
Displayed on the display 15 of the personal computer 16 is an operating picture for adjusting the focus and so on using the window system of the personal computer 16, which is controlled by operating an input means, such as a key board 18, a mouse 19, a track ball or the like. Control command communication is performed between the personal computer 16 and an SEM control unit 14 and transmission of image data is performed between the personal computer 16 and the image memory 13. A standard interface for personal computers, such as RS-23C, SCSI (Small Computer System Interface) or the like, is often used for connecting these units.
There are some cases where an X-ray analyzer, such as an energy dispersive X-ray spectrometer (EDX), is connected to a scanning electron microscope to perform material analysis of a sample using an X-ray generated when the sample is irradiated with the electron beam. A personal computer or a workstation is also used in the X-ray analyzer to display control items of the X-ray analyzer or an analyzed result. Therefore, in a case of a scanning electron microscope combined with an X-ray analyzer controlled by a personal computer, there are provided two personal computers.
The SEM control means 14 serves to change parameters associated with the image of the sample 5 on the basis of received control signals or commands into which the computer 16 converts operating signals input by manipulating the input means. Such parameters are a focal length of the focusing electron lens system, which can be changed for focusing control by adjusting currents flowing through lens coils of the focusing electron lens system, astigmatism of the electron beam, which can be changed for astigmatism correction by adjusting currents flowing through the stigmater coils, a deviation of the axis of the electron beam from a correct axis thereof, which can be changed for making both the axes coincident with each other, i.e., axis alignment by adjusting currents flowing through the alignment coils, brightness and contrast of the image, which can be changed by adjusting the bias and gain of the detector device with the bias and gain controller, acceleration voltages, scan rates, which can be changed by changing currents flowing in the deflection coils, an amount of movement of a field of view of the sample 5, which can be changed for selecting the field of view by changing currents flowing through the image shift coils or moving the sample stage, an emission current which may be changed, and so on. The starting/stopping of scanning, starting to take a photograph, the setting of the sensitivity of a photographic film, the measuring of a length between arbitrary positions on the image are also performed under control of the SEM control means 14 on the basis of the control signals into which the computer 16 converts the operating signals input by manipulating the input means.
The prior art described above is disadvantageous in that the response time cannot provide a pleasant operating condition because processing for the window system and the communication control from operation of the input means, such as a mouse, to execution of control of the hardware takes a long time period.
A case where the focus is adjusted by controlling the focusing electron lens system while observing an SEM image will be described below as an example. On the display 15 of the personal computer 16 shown in FIG. 2, a slider (scroll bar) for adjusting focus is displayed through the window system, and adjustment of the focus is executed by dragging the knob of the slider through a mouse operation (an operation of moving the mouse while pushing the button). The OS (Operating System) generates an event in the window system of the personal computer 16 by the movement of the mouse 19. With the event, the window system executes processing to move the knob on the display 15 and issuance of an event to an application program. An SEM control program of the application program judges the event as adjustment of focus and issues a control command to the SEM control unit 14. The control command involves an alignment of data in a format appointed between the personal computer and the SEM control unit, and processing is required for conversion of the format. Further, the SEM control unit 14 decodes the received command to recognize the command as a request for control of the electron lens system 2, and changes the setting of the irradiation system controller 9 of the hardware. As the focus value, i.e., focal length, of the focusing electron lens system is changed, an SEM image according to the changed focus can be obtained. The processing time to this point becomes several hundred seconds, and the response is not fast enough to repeat the above processing until an image having an optimal focus is obtained, and accordingly a pleasant operating condition cannot be obtained.
Further, in order to execute image processing, such as filtering, of image data using a personal computer, it is required to transmit the image data from the image memory 13 to the memory in the personal computer 16. The transmission speed of the image data is approximately 1 mega-byte/second in effective speed when on SCSI interface is used. The time for transmitting image data of 4 mega bytes having vertical and horizontal resolutions of 2000 dots each and tones of 8 bits becomes approximately 4 seconds. However, it is typical for an operator to complain about his operating environment when the processing time from the start of an operation of data transmission to completion of the processing exceeds 1 second.
Where an X-ray analyzer is connected to a scanning electron microscope, a personal computer or a workstation is used for controlling the X-ray analyzer and displaying its analyzed result. Therefore, two personal computers are provided including the personal computer for the scanning electron microscope. Two input/output units are also provided. Accordingly, the operator cannot avoid the need to operate two sets of input means, which results in an environment difficult to operate in.